This invention relates to optical amplifiers of the kind which have a so-called loop mirror formed by a loop of optical fiber with its ends connected to a 3 dB coupler. The invention is applicable especially, but not exclusively, to optical fiber amplifiers in which the loop mirror comprises a rare earth-doped fiber.
As explained in U.S. Pat. No. 5,757,541, which issued May 26, 1998 and named B. G. Fidric as inventor, optical amplifiers are known in which an optical signal to be amplified is passed through an active fiber together with pump energy from a separate source, such as a laser diode.
When discussing the prior art, Fidric explained that, whether the optical signal and the pump energy were both supplied to the same end of the active fiber, or to opposite ends of the active fiber, the known amplifiers exhibited non-uniform, non-symmetrical longitudinal pump excitation. Also, despite the use of isolators at the input and the output, backreflection resulted in a certain amount of pump energy reaching the input source.
Fidric proposed to overcome these problems by means of a loop mirror arrangement comprising an active fiber with its ends connected to two ports of a four-port 3 dB coupler. The input signal and pump energy supplied to the other two ports were split into two equal parts by the coupler and propagated simultaneously clockwise and counterclockwise around the loop. The input and output signals were applied to, and extracted from, the coupler by means of a circulator. The fiber was polarization-maintaining so that the states of the signals propagating through the loop mirror were maintained. Maintaining the polarization states was preferable to ensure that the amplified signal was not contaminated by the residual pump light. According to Fidric, this could be achieved, in the alternative, by means of a polarization controller in the fiber loop, but that is debatable.
The amplifier""s noise figure and gain depend upon the extent to which the xe2x80x9cforwardxe2x80x9d and xe2x80x9cbackwardxe2x80x9d ASE in the fiber are equal, so it is desirable for the 3-dB coupler to split both the pump energy and the input signal precisely into halves. Consequently, the coupler must be capable of providing the same 50xe2x80x9450 splitting over a range which includes both the pump energy wavelength and the input signal wavelength.
It is usual to pump an active fiber at different wavelengths, depending on the application. For example, when power output is the main consideration, a pump wavelength of 1480 nm is preferred. When noise is the main consideration, however, it is preferable to use a shorter wavelength, such as 980 nm, because the amplified spontaneous emission (ASE) produced by the active fiber is less at that wavelength. Fidric""s proposed amplifier will not be entirely satisfactory when used with the shorter pump wavelength because, at present, a coupler capable of providing precise 50xe2x80x9450 splitting over a range from 980-1600 nanometers is not available. Even if pump energy with a wavelength of 1480 nm were used, the problem would persist, though to a lesser extent.
An object of a first aspect of the present invention is to eliminate or at least mitigate the above-described disadvantages and, to this end, there is provided an optical amplifier having a loop mirror comprising an active fiber and a coupler. Preferably, the pump energy is coupled into the fiber without passing through the coupler.
In one embodiment of the invention, an optical amplifier comprises a loop mirror in which a four-port 3 dB coupler has an active fiber connected between second and third ports, respectively, of the coupler, the coupler having a first port to receive an input signal for amplification, the output signal being provided at either the first port or a fourth port, and at least one pump means coupled between the coupler and the active fiber for injecting pump energy into the active fiber so as to copropagate therein with the input signal, the arrangement being such that the coupler will split the input signal into two equal parts which will propagate in opposite directions within the active fiber.
Preferably, two identical pump means are provided, adjacent respective ends of the active fiber, for injecting pump energy into the fiber in opposite directions, and the input signal and output signal are coupled to and from the first port by way of a circulator having its bidirectional port coupled to the first port. An advantage of using two identical pump means is that the xe2x80x9cforwardxe2x80x9d and xe2x80x9cbackwardxe2x80x9d ASE in the active fiber can be equalized more readily. Alternatively, the amplifier may comprise a first isolator for coupling the input signal to the first port, a second isolator for coupling the output signal from the fourth port of the coupler, and a polarization controller in the loop, in series with the active fiber, for adjusting the polarization of the signal in the loop so that the output signal will appear at the fourth port.
The or each pump means may comprise a wavelength multiplexer and a pump energy source, such as a laser diode. A second aspect of the invention concerns automatic gain control in rare earth-doped fiber amplifiers. It is desirable to be able to maintain the amplifier gain at a constant value over a wide range of input powers. In an article entitled xe2x80x9cGain-clamped Fiber Amplifier with a Loop Mirror Configurationxe2x80x9d, IEEE Photonics Technology Letters, Vol. 11, No. 5, May 1999, Kyo Inoue explained that it is known to clamp the gain using optical feedback. According to Inoue, drawbacks of such gain-clamped amplifiers include the fact that they cannot be used for signals around the oscillation wavelength because such light enters the feedback loop and is not amplified efficiently, and laser oscillation light appearing at the output is an obstacle to system application.
Inoue proposed overcoming these drawbacks by means of a gain-clamped fiber amplifier in which the laser cavity is formed by a fiber grating and a loop mirror that comprises a loop of erbium-doped fiber connected to two ports of a 3 dB coupler. The grating and the pump energy are supplied to a third port of the coupler and the input signal is supplied by way of a circulator to a fourth port. The output signal is extracted via the circulator. According to Inoue, the signal light passes through the EDF without entering the laser cavity, even when its wavelength is close, or identical, to the oscillation wavelength.
This approach is not entirely satisfactory because it requires all of the components to be polarization-maintaining, which makes the amplifier complicated and expensive to manufacture. Another disadvantage is that available wavelength is limited because the wavelength of the fiber grating is within the useful wavelength of the amplifier.
According to the second aspect of the present invention, there is provided an optical amplifier comprising a loop mirror formed by a four-port 3 dB coupler and a length of active fiber having its ends connected to first and second ports, respectively, of the coupler, the coupler having a third port to receive an input signal for amplification, such that the coupler will split the input signal into two equal parts which will propagate in opposite directions within the active fiber, the output signal leaving the coupler via the third port, and at least one pump means coupled between the coupler and the active fiber for injecting pump energy into the fiber, further comprising means responsive to a portion of the amplified signal for reflecting into the loop mirror a selected wavelength that is outside a normal operating range of the input signal. An attenuator may be provided between the reflecting means and the coupler for adjusting the amount of the amplified signal reflected back into the loop mirror.
The reflecting means may comprise a grating, such as a fiber Bragg grating. Where the loop comprises an erbium-doped fiber, the selected wavelength may be, for example, 1525 nm. Because amplifiers embodying the present invention supply the pump energy directly in to the loop, i.e. without passing through the coupler, it is possible to use pump energy having a wavelength of 1480 nm for high power applications and 980 nm for applications requiring a low amplifier noise figure. The pump means may conveniently comprise a wavelength multiplexer connecting a pump energy source, for example a laser diode, to the loop.
In accordance with another embodiment of the invention there is provided an optical amplifier having a loop mirror including an active fiber and a coupler, optically coupled to a linear amplifying section. The linear amplifying section amplifies an input optical signal, while simultaneously producing amplified spontaneous emission that is used to further pump the amplified optical signal passing through the active fibre in the loop mirror.